Partially braked cornering maneuvers present a problem of driving dynamics within the limit range of stability which is not sufficiently under control of conventional anti-lock control systems. Above all, vehicles with a short wheel base tend to suddenly oversteer in such braking situations. Oversteering is often not readily foreseeable by the driver and cannot be easily stabilized in real-life driving situations. FIG. 1a explains the oversteering problem during a partial braking operation. Referring to the FIG. 1, a vehicle 100 moves from the bottom upwards along curve 101 (solid line) Accompanying the cornering maneuver of vehicle 100 is a rotation of the vehicle around its vertical axis. Together with the moment of inertia of the vehicle, this rotation around the vertical axis causes an angular momentum. This angular momentum around the vertical axis must also be braked, together with the braking operation of the vehicle along curve 101. In FIG. 1a, it is assumed that the braking operation of the vehicle along curve 101 comes on in the bottom position illustrated. The braking operation brings about a reduction of the speed v(1) on the circular path 101 and thus, also, a reduction in the speed of the angle of rotation of the vehicle around its vertical axis. The angular momentum, however, causes the vehicle to try to keep up the original rotation around the vertical axis and thus to rotate more strongly than required by the diminishing vehicle speed, thus causing a swing-in tendency. As, within the partial braking range (all wheels applied by the same braking force), this tendency is not counteracted by any countering moment (e.g., due to varying actuation of the brakes) it is impossible to prevent the vehicle from slightly swinging inwards, since the lateral guiding forces of the wheels build up only as forces reacting to the occurrence of the inward swing because of the then increased oblique motion angles. The vehicle thus having reached an increased oblique motion angle xcex2(2), there is also an increase in the steering moment on the front axle with the steering wheel position remaining unchanged, and the vehicle steers along a narrowed course. This narrowed course is represented by the broken line 102. This may result in instability and in swinging around the vertical axis if, previously, driving took place within the limit range of stability.
It often happens that ABS comes on too late in a situation as described above. ABS is a wheel-individual type of control with comparatively insensitive threshold values with regard to the respective wheel slip. In FIG. 1a, ABS would come on no sooner than in the top position when the vehicle would already have built up a considerable sideslip angle. However, then, ABS is no longer able to stabilise the vehicle.
It is an object of this invention to provide a method and a device for identifying an unstable braking operation and for controlling the braking pressure within the partial braking range, by means of which it is possible to identify an unstable braking operation or rather to prevent it as far as possible.
This task is solved by the characteristics of the independent claims. Dependent claims are directed to preferred embodiments of this invention.
The inventive identification of an unstable braking operation does not just perform a selective check of the running behaviour of any individual wheel, only. What is done rather is the comparison of the running behaviour of a left-hand side wheel with the running behaviour of a right-hand side wheel and/or there takes place an observation and evaluation of the time variation of the running behaviour of a wheel, preferably on the inside of the corner, over a longer time. On the basis of these critera it is possible to identify an unstable braking operation so as to enable suitable countermeasures to be initiated.
An unstable braking operation such as indicated above being identified, it is possible to initiate as a countermeasure the reduction of braking pressure on at least one wheel on the inside of the corner. This causes the wheel on the outside of the corner to be braked more strongly so that the swing-in moment caused by the angular momentum of the vehicle is counteracted by a swing-out moment caused by the unequal braking forces on the wheels on the inside and outside of the corner. It is thereby possible to prevent swinging-in of the vehicle as shown in FIG. 1a (transition from solid line 101 to broken line 102), the vehicle driving safely along the provided curve 101.
FIG. 1b shows an exemplary partial braking situation in the limit range of the corner where ABS intervention comes on too late. The top graph shows the vehicle speed by means of curve 103 and any wheel speed by means of curve 104. The bottom graph shows the wheel braking pressure by means of curve 105 and, in comparison therewith, the braking pressure in the tandem master cylinder by means of the broken-line curve 106. The wheel slip values being sufficient to activate ABS, the vehicle has already reached a very high sideslip angle. The braking pressure reduction then coming on on all wheels does not bring about any stabilising countermoment as, generally, no big difference develops in the braking force on one axle. If a very high sideslip angle has built up (the beginning of the vehicle""s standing obliquely) ABS reduces the wheel braking pressure to almost 0 bar, typically at time T3 of the represented example, without the wheels being able to reaccelerate. This is no earlier possible than when the vehicle has swung by more than 90xc2x0 around the vertical axis, then skidding (as of time T4). Thus, ABS is not able to prevent the vehicle from swinging in towards the inside of the corner. Wheel-individual anti-lock control is not expedient in this case as slip is only caused by the high oblique motion angles of the wheels, yet not by overbraking. Likewise inexpedient is consequently the effort of counteracting the slip plunge of the wheel by means of pressure reduction; it rather may even be hazardous because the vehicle, then suddenly underbraked after a rotation by more than 90xc2x0, may skid along the running direction of the wheels (time T4).